For pipeline landfalls (shore approaches), the trenchless Direct Pipe® method is gaining in popularity. The term ‘shore approach’ means the installation of pipelines from water to land or from land to water. Depending on the perspective and function of the pipeline, it is also referred to as landfall, sea outfall or sea intake.
The Direct Pipe technology was developed by Herrenknecht more than 10 years ago and combines the advantages of microtunnelling and horizontal directional drilling (HDD) to create an efficient installation method for underground pipelines when conventional methods reach their limits.
Since its launch in 2007, more than one hundred drives have been realised using Direct Pipe, including in the installation of oil, gas, water and saline brine pipelines. In contrast to the multistage HDD method, in Direct Pipe the pipe string is inserted during the drilling process, which has several advantages in terms of efficiency and control of the construction process.
This presents an interesting alternative for owners, clients and planners of pipelines, especially for often time-critical shore approach projects. Whether Direct Pipe or another method is the right choice for a pipeline project must be determined on a case-by-case basis through careful analysis of geological, local and other conditions affecting the undertaking.
Choosing Direct Pipe
Direct Pipe is used for crossing key sections in a project and of the works undertaken, 90 per cent are below bodies of water. Unlike HDD’s multistep process, with Direct Pipe the insertion of the pipeline is done in a single step while simultaneously creating the borehole.
In contrast to conventional pipe jacking or microtunnelling, the method pushes in extended pipe strings, prefabricated and welded together on land, up to several hundred meters in length. This ensures the high demands regarding process integrity and quality assurance common in pipeline construction can be met.
Direct Pipe does not require a deep launch shaft. Generally, a pit sloping slightly downward to match the entry angle of the borehole is sufficient. The drive always takes place from shore, where a pipe thruster pushes the pipeline from the shore toward the water.
Direct Pipe projects have accomplished lengths of up to 1,900 m with diameters of 30–56 inches (762–1,423 mm). Further, new approaches with compact drive units and specially developed jet pumps for muck removal allow longer drive lengths even for diameters below 36 inches (915 mm).
Direct Pipe has some characteristic advantages that are particularly important for crossings in ecologically sensitive coastal areas. The microtunnelling and pipeline installation is completely underground and any environmental effects due to hydraulic fracturing can be avoided.
At the tip of the pipeline, a modified microtunnel boring machine (MTBM) bores through the ground. After the pipeline has been completely inserted in the ground, the Direct Pipe MTBM is floated from the seabed to water surface and retrieved by crane, with no further offshore work required.
In individual cases, if the timing is appropriate, recovery by a pipelaying vessel is conceivable, which can then seamlessly begin further laying the offshore pipeline. The Direct Pipe MTBM is similar in design to microtunnelling machines used for pipe jacking.
The direction of the drive is controlled by a navigation system, which, in conjunction with the MTBM’s articulation joints, also allows curved alignments.
The gyro-based navigation system, used in combination with an electronic water levelling system, achieves greater targeting precision with just a few centimetres’ deviation. In contrast to walkover detection, Direct Pipe navigation can easily be used underneath the seabed, providing a clear advantage in shore approaches.
The pipeline is inserted into the borehole using the pipe thruster, which is a permanent part of the Direct Pipe system. This continuously pushes the pipeline forward and can, if necessary, pull it back together with the MTBM.
As a world market leader in mechanised tunnelling technology, Herrenknecht has extensive expertise in pipeline construction. Approximately 380 km of pipeline is installed annually using Herrenknecht technology.
Based in Schwanau, Germany, the company supplies not only tunnel boring machines and peripherals, but also acts as a service partner to its customers during the entire project.
For more information visit the Herrenknecht website.
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